The adrenocortical hormone (ACTH) stimulates steroidogenesis by increasing the rate of formation of pregnenolone in adrenal cortex mitochondria. Protein synthesis has been implicated in this process. Present evidence indicates that ACTH does not stimulate protein synthesis but rather mediates the transformation of a rapidly turning-over protein to a form which is active in the stimulation of pregnenolone synthesis. Since ACTH stimulates the formation of 3',5'-cyclic nucleotides, which in turn stimulate protein kinases, this active form is most likely a phosphoprotein. We have detected in adrenal cortex mitocondria a system which results in the phosphorylation of an endogenous protein(s) as revealed by gel-electrophoresis. The phosphorylation is increased by ACTH and decreased by inhibitors of protein synthesis. Currently we are determining the properties of this system and attempting to implicate it in pregnonolone synthesis.